There are many potential applications where a renewable heat source will be combined with an existing or new boiler. The latter serves as a supplemental heat source when and if the renewable heat source is unable to supply water to the heating load at a suitable temperature.
The control concept is simple: Draw heat from the renewable heat source, its associated thermal storage tank or both whenever it can supply the heating load, and when it can’t, supply the load from the auxiliary boiler. The renewable heat source becomes the “lead” stage for heat injection to the distribution system while the auxiliary boiler becomes the “lag” stage. A piping schematic that could be used for this concept is shown in Figure 1.